At present, very little is known about the regulatory events associated with proliferation and differentiation of human lymphoid precursor cells. A first major goal of the proposed studies is to expand and clarify the current knowledge of the basic immunobiologic features of normal and leukemic B-cell precursors and biphenotypic lymphoid precursors (Specific Aims 1 and 2). To this end, we will examine the effects of biochemically purified B-cell growth factors (CGF), recombinant (r) interleukin (IL)- 1,rIL-2, rIL-3, r-IL-4, rIL-5, and rIL-6 on in vitro proliferation/differential of leukemic B-cell precursors (BCP) and biphenotypic lymphoid precursors from the bone marrows of acute lymphoblastic leukemia patients as well as their normal counterparts from fetal hematopoietic tissues including a fetal liver BCP cell line and a fetal liver biphenotypic lymphoid precursor cell line. Effects of growth factors on proliferation will be studied using in vitro colony assays and multiparameter quantitative DNA flow cytometry. In vitro differentiation will be monitored by immunophenotyping and in the case of fetal liver cell lines also by Southern blot hybridization analyses. Equilibrium binding assays and Scatchard analyses will be performed to characterize the growth factor receptors on normal and leukemic lymphoid precursor cells/cell lines. We will further explore the effects of defined biomatrix products and fetal bone marrow stroma on the growth factor responses of normal/leukemic BCPs/biphenotypic lymphoid precursors. A second major goal is to study the interactions between functionally important antigens on the surface of normal/leukemic BCPs/biphenotypic lymphoid precursors using monoclonal antibody heteroconjugates Specific Aim 3). The bioactivities of heteroconjugates will be analyzed using sensitive assays of signal transduction by measuring their effects on cytoplasmic calcium concentration, colony assays by measuring their effects on the proliferative responses to growth factors, and in vitro differentiation assays by studying their effects on growth factor induced differentiation. We anticipate that the proposed basic research studies will help us to elucidate which of the hematopoietic growth factors regulate the proliferative activity or differentiation of BCPs/biphenotypic lymphoid precursors and also to determine if differences exist between normal and leukemic BCPs/biphenotypic lymphoid precursors relative to their growth factor receptor profiles or to the functional properties of their surface antigens. Such basic knowledge concerning lymphoid precursors is not only important for a better understanding of some of the earliest events in the development of human lymphoid cells, but it may also help us to better understand the pathogenesis of and design potentially more effective treatments for diseases such as acute lymphoblastic leukemia or severe combined immunodeficiency.